Asymmetrical dual PTCR package for motor start system

ABSTRACT

A motor start and run system for a single phase motor provides a predetermined period for starting the motor during which power is delivered to a start winding of the motor. It also provides a time delay after the motor is deenergized before the motor may be restarted. These two control times are essentially determined by thermal time constants associated with a package having two PTCRs. These two PTCRs are thermally coupled and have different anomaly temperatures. A dual-PTCR package is also disclosed for use in said motor start system having provision for strongest thermal coupling with the environment for the PTCR having the higher anomaly temperature.

United States Patent Moorhead et al.

[4 1 Apr. 15,1975

[ ASYMMETRICAL DUAL PTCR PACKAGE FOR MOTOR START SYSTEM Faxon 323/69 X Stachan 3l8/22l E [75] Inventors: William L. Moorhead; Manfred r Kahn, both of Williamstown, Mass. Pmlmry Emmlrler j' ASSISIGH! ExammerPatr|ck R. Salce [73] Assignee: Sprague Electric Company, North Attorney, AgenJ, or Firm Connolly and Hutz Adams, Mass.

[22] Filed: Jan. 2, 1974 [57} ABSTRACT [21] Appl. No.: 430,337 A motor start and run system for a single phase motor providesa predetermined period for starting the [52] us Cl 338/22 W100 3 l7/234 A motor during which power is delivered to a start wind- 338k} 338/535 338/320 ing of the motor. It also provides a time delay after the [51] Int. Cl llOlc 7/04 motor deenerglzed befmle the motor "F be re- [58] Field of Search 317H00 234 A. 338/22 R started. These two control times are essentially d eter- 338/22 SC 23 24 25 204 mined by thermal time constants associated with a b i e 5 package having two PTCRs. These two PTCRs are thermally coupled and have different anomaly temper- [56] References Cited atures. A dual-PTCR package is also disclosed for use in said motor start system having provision for stron- UNITED STATES PATENTS gest thermal coupling with the environment for the 2,769,07l lO/l956 Ward 338/22 PTCR having the higher anomaly temperature. 3.307,l67 2/l967 Race l l l 317/41 3,586.93) 6/1971 Buiting et al 318/22! E 3 Claims, 5 Drawing Figures s Q 1 60 "IQE s \\L\ 12 H Q10 ASYMMETRICAL DUAL PTCR PACKAGE FOR MOTOR START SYSTEM BACKGROUND OF THE INVENTION This invention relates to a thermistor package for use in an ac motor start and run system. In such systems. the motor has a main winding. and a start or auxillary winding. During start both windings are energized. During normal run conditions the main winding is always energized whereas the start winding for certain motor designs is decnergized and for other cases the running current in the start winding is supplied at a re duced value. Potential relays. current relays. and centrifugal switches have been traditionally used to provide this function. These devices tend to be expensive and unreliable.

It is known to use a thermistor such as a positive temperature coefficient resistor (PTCR) as a time delay element in a motor control system for automatically determining the start period duration avoiding the shortcomings of the aforementioned traditional devices. The PTCRs of interest herein are of the type having an anomaly temperature above which the electrical impedance of the PTCR is several orders of magnitude greater than it is for temperature below the anomaly temperature.

A second function that a motor control system desirably performs is that of preventing restart for a period after deenergization to allow for instance a time for the motor driven compressor system to equalize pressure. or to allow time for an overheated motor to cool to a safe temperature. Electromechanical timers have been used to provide this function as have temperature responsive elements or switches that are thermally coupled to the motor.

The conventional system using a single PTCR in series with a motor start winding to accomplish the start function. has the shortcoming of a non-definitive restart function such that a premature attempt to restart the motor has the effect of providing an abnormally short start period that in turn may result in improper or sluggish starting. Furthermore. in many motor start systems it is desirable to keep both the start and main windings disconnected from the line during the restart delay period. which the single PTCR system cannot do without the addition of more components and additional circuit complexity.

It is therefore an object of this invention to provide a simple low-cost motor control system.

It is a further object of this invention to provide a motor control system capable of disconnecting both start and main motor windings from the power line during a predetermined nonrestart period.

It is a further object of this invention to provide a motor control system having a definitive restart characteristic.

It is a further object of this invention to employ in a motor control system a package of PTCRs whose thermal characteristics determine the motor start period and the delay to restart period.

A PTCR package comprises a first and second PTCR having thermal coupling therebetween. The first PTC'R has a lower anomaly temperature t an the second PTCR. The thermal coupling between PTCRs may be achieved by sandwiching a metal plate therebetween or alternatively by sandwiching a layer of insulating material therebetween. The second PTCR is provided a stronger thermal coupling to the package emironment than the first PTCR. The package of this invention is ad\antageously employed in an ac. motor start and run system whereby the second PTCR is electrically connected in series with the motor start winding. The first PTCR is connected to the input of a switching control means. that may be a solenoidal relay having hysteresis. such that activation ofthe switches may not be initiated when the first PTCR is in the high impedance state. and such that the two motor winding circuits are connected to a power source only when the first PTCR is initially in a low impedance state. and so that the motor winding circuits are held closed when the first PTCR is heated above its anomaly temperature to a high impedance state by thermal energy from the second PTCR.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I shows a schematic diagram of a motor start and run system employing a relay and a dual PTCR package of this invention.

FIG. 2 shows a graph of two characteristic volt-amp curves of a PTCR and their necessary relationship to a load line. according to the principles of the present in vention.

FIG. 3 shows a schematic diagram of another motor start and run system employing a dual PTCR package of this invention.

FIG. 4 shows in cross section a PTCR package of this invention having a selective heat removal means.

FIG. 5 shows in cross section a second package of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. I shows a diagram of a motor start and run system of this invention. A first PTCR I0 is shown having a thermal connection 1] with a second PTCR l2. The first PTCR has a lower anomaly temperature than the second PTCR I2. It is well known to make PTCR bodies having different anomaly temperatures. as shown in the Effect of Heat-Treatment On the PTCR Anomaly in Semiconducting Barium Titanate. by M. Kahn; pages 676680. American Ceramic Society Bul- Ietin Vol. 50. No. 8. August i971. Winding is the start winding of a split phase a.c. motor having a main or run winding 22. The start winding 20 is connected in series with the second PTCR 12. An electromechanical relay has a coil connected in series with the first PTCR I0. When an ac. power source (not shown) is connected to terminals 1 and 2. and when a manual switch is closed. power is applied to the relay coil 30 and the series connected PTCR 10. Since at start. the PTCR I0 is at or near room temperature. it is in its low impedance state and an initial current flows in the relay coil 30 causing switches 31 and 32 to close. A mechanical connection 33 represents a part of the magnetically activated moving member of the relay linking the coil 30 and the switches 31 and 32. This initial current is predominately limited by the resistance of the relay coil 30 and is low enough. that accounting for the thermal properties of PTCR I0 and particularly its thermal coupling with the environment. the initial starting current is not large enough to cause the PTCR 10 to reach its anomaly temperature and its high resistance state dur ing this period when the motor is starting. Thus PTCR 10 cannot self heat.

Sell heating ol P'lCR lll is a\oided h using a relay whose coil impedance is high enough in series with a PTLR whose characteristic power dissipation is high enough that at room temperature the \oltage across it will not he sufficient to cause the PTCR to s\\ itch to its high impedance tate llii condition is illustrated hy the graph of FIG 2 wherein LUHL 1 represents a PTCR \olt;mipere characteri tic t pical l'or PTCR 10. with no external heat applied. mine 2 represents the \olt-anip characteristic of the same PTCR with a gi\ en amount of heat being applied. as from the second thermall coupled PTFR 12 of FIG I, and the straight line 3 represents the load line of the series impedance. cor responding in HQ. l to the impedance ol the relay coil The load line 3 has a slope corresponding to the relay coil impedance and it intersects the \oltage axis at point h where the upply \oltage is shown as. V This load line should also account for any additionai impedances that may he in series with the PTCR 10.

Note that there is only one intersection of the load line 3 with LUHL 1. namely point l4 at which the olt age appearing across the PT(.R H] is relati ely small and the current I through PTCR It is relatitely large Howc er. when e\ternal heat is sittttlltancously applied to Plt'R H). the PTCR I is characterized hy cutie 2 in HO. 2 and. a second stahle operating point. namely 15. is achie ted at which the voltage I across the PTCR II) is relati ely large and the current l, in he eir cuit is relatnely small. hi the practice of the it 'tant in \ention it is essential that there he oiil\ one stahle per ating point (points [4 and I5) associated with each oi the two can es tcur\ es 1 and 2t and thus the trio steady state conditions of PTVR lll, narncly twtcrnally heated and not csternally heated.

When the relay is initially actitated and nitchcs and 32 are first closed the second P'U'R [2 in series connection with the motor start winding is suppiied powei from the a c power source l iltei isc. power is supplied to the motor inain winding 22, Since the sec ond Plt R 12 is nitially :it or near ro m temperature. it is in its low impedance state and liitgc current lions in the \lllil aiding 21!. Thus. the moto tarts with hoth windings It? and Ill c nnected o the sa'iurce o! a t power A short time after switch 50 is cl sed normally ahiu i second the rnotor has essentially nchieted l'lttllllll iiill running peed 'lhc urrent through the low iniped ancc tart it indiiig is high enough to heat PTCR 12 a that it reache its anomaly tcinpeiaturihen the niotoi is upcctcd ti hu achieteil l'ull running sp ed itt this point the mpedance oi PTCF l2 ahruptly incret es hi scteial orders ol magnitude and reduces the current in the start it inding aceoitliiiglt The FTC?! ll continues o ell heat sufficiently to maintain it temperatur aho r lllt' anon al} temperature it upplies thermal en ergy iiu the thermal coupling ll iv th first PHl". lit causing the teinptratiire of the; first PlT'R l" to rise and remain aho\ e it anomaly temperatuic. 'l his n turn reduces the current in the relay coil Fil to a le\ cl l;.,. that i below the relay lillitliftltl'titll hut ahoie the drop outcurrent l'hus the relay remain aetiated during normal ran conditions The inherent hy teresis ot' the relay is therehy seen to e used here (tther switching t ontriil means may alternatiiely he used. having a :neittoii .ilttl logic tapal iltt} to acluet e the same result. t aiticiilarlx those using siiicon controt rectiliers and integrated seniict' n'tuetot logic detues "ill It can he seen that it for any reason power is interrupted. for ewtnplc at the power source itself or by opening switch 50 or by opening the series relay .30 and l l (R It) circuit. then the relay current ill fall to Iero. and switches 3| and 32 wilt open. If power is immedi ately or soon thereafter restored to the circuit. the current in the relay coil 3|) is limited h the hot high inipedance of PTCR 10 to a leiel below the pull-in current of the relay. and the motor will not start. The motor is preieitted from starting until the temperature of Pl(R l0 falls below its :inoinah temperature This time delay f r re tart is designed to he long enough for the particular :ao or emplm ed to cool to a sale temperature lttihllllllllli that power was initially returned in re sponse to a fault and the motor hating merlieated. A typical delay to res in time is on the order i three minutes The wire link shown iietween terminals 3 and 4 may he removed and ..i heat seitsitiae switch that is thermally connected to th motor may he eiectrtcally connected hetween termn .ils t and i to accomplish this func ion. Such heat switches may he a third PII'R (not shown. hir

shown Many oil, t -I.ll'ltlllitlts ol thi tfll' relay mat proyided wi h a hird n rtilalh. open contittts toulncted litween lctmlilli'y 3 and 9 such that the relay is held a tirated thcrehy during normal run conditions. A resi tor may he added in series with this third set of contacts the relay holding current is determined in these ways without. dependence on the particular \alue ot'liot high impedance ot'the P'lC'R ill. Alternati' el a resistor may lie permanently connected in parallel with PTFR l" to help guarantee that the relay will see the proper aluc oi' holding current or to ;3.uarant.ec a shapc oi the equivalent \olt-ampere LLl1\C plot ides only one ll'llt rsect s ith the load line at all internal tein 'ici'atur s' oi Hill It}.

Aiso. w. ill a tutor hat motor i employed a tart capacitor may he suhstituteil tor the link shown he ween terminals 7 and it iii serie with the start winding 20. it also may he tie' it'u lc to additional!) connect 1 ittiillcr capacitor to terminals 8 and 9 in parallel with the second P'HR I2 s that during run conditions a low i alue tpiadriture current will llow in the start winding and contrihutc to tann ng owei and smoothness of itiotor optratiein as ls iii-ti inowii to the art t further :iltc-rnatii e lllit'lhCs ii'ioditication otthc impedance in series with PTCR iii. A resistor may he added. for example in place l the win iinlt hetween points 3 and 4 ol i itl l. n or der o l lit'd the load liitc sl pe a shown in i'lti ind thus t mee the necessary rclatiw relationships heti-rr-en thtiolt imp character and 1 in its prmiously (le :lirc switrh lnot aii. possihlc. The

istics tcui t's l lt Another motor ta 't 'hown H PH 3 cittg imilar t that ol" l l i. E t o ept hat rip a separated and isolated at, power sourciupplying the series relay coil 36 and PT('R Il 'lhis is :iccoirtplishcd hy connectlug a trtiiistiitiiier 4!) n uh the print-at to the first power series coil and l R N? ti cur i sw itch 33 is sh t a shooting the l l t R. ltl llttsi a ii' how hetd actuated during no mal nn c nditi ns 'l he trans tornier 40 is picl'eial ly i wilt step down type so that h c st oi the relay "llitl li't is til aia'. lit. reduced. The

i sources til t t-ttii the :le i con the: Fl li'i l'or marnp a con ource .ind he s eont ai. to tie relay may l e isolated po nection tic-tween the nection between terminals 5 and 9. that may simplify and make less expensive the PTCR's package assembly in the aforementioned motor start systems it is preferred that upon interruption of power to the control circuit that the second PTR 12 cool to a temperature below the anomaly temperature thereof before the first PTCR l cools to a temperature below its own anomaly temperature. in this manner. the P'l'CR It) will reach a low impedance state and permit closure of the relay and restart of the motor after the PTCR 12 has reached its low impedance state. It is accordingly preferred to use a dual PTCR package wherein the second PTCR (e.g.. PTCR 12) has a stronger thermal coupling to the environment than the first PTCR. However. the circuit can be designed with a dual PT(R package wherein the two PTCRs have the same thermal coupling to the environment.

The time difference between these two events is preferably long enough for the temperature of PTCR 12 to fall near its quiescent steady state room temperature condition. Thus upon restarting. the start time period during which the high current flows in the start winding is always of a uniform length and a reliable and definitive delay-to-start system is realized. Furthermore. premature attempts to restart the motor may never result in an abnormally short start time. as is the case for conventional PTCR start systems.

ln H6. 4 is shown a dual PTCR package suitable for use in the motor start system of the present invention. A first PTCR element [0 is contacted by and in this embodiment bonded to two metal plates having extensions 4 and 5 serving as electrical terminals therefor. A second PTCR element 12 is contacted by two metal plates having extensions 8 and 9 serving as electrical terminals thereof. An electrically insulating thermally conducting part ll serves to thermally couple the PTCR element 10 with the PTCR element 12. and may consist ofa thin wafer of ceramic materialv These parts may be bonded together and further attached to a metal base 60. Alternatively the PTCRs and metal plates stack may not be interbonded. but rather be mounted and held in compression by adding an insulating block (not shown) between plate 4 and the roof of housing 61, or by other means. A metal housing 61 contains the PTCR elements and is attached by screws 62 and 63 to the base 60. Holes 65 and 64 in base 60 provide means for screw mounting the PTCR package in thermal contact with a flat heat sinking surface that is maintained at room ambient temperature The first PTCR 10 has a lower anomaly temperature than the second PTCR l2 and the second PTCR is in stronger more direct ther mal contact with the package environment so as to cool at a faster rate. Thus this dual PTCR package is seen tp be uniquely appropriate for use in the motor start system of this invention For motor control systems where there is no requirement for switching open both sides of the line. simplification of both the circuit and the PTCR package is possiblc. For example. in Fl(i. I switch 32 may be climi noted and replaced with a short circuit. It is seen that one lead of PTCR 10 is in this instance always con nected to one lead of PTCR [2. Thus as shown in FIG. 5 the PTC'R's l0 and 12 may be packaged with one common lead 7-8. The two other leads 73 and '75 are of course independent. Also illustrated in this embodiment of the package of this invention is the addition of an insulating layer 71 that may be a thin plastic film such as 0.00l inch polyethyleneterephthalate. Such a film provides electrical isolation between the PTCRs. particularly lead 73. and a metal base 70. This thin but relatively broad film also serves as an efficient thermal coupling between the PTCR l2 and the base 70.

What is claimed is:

I. A PTCR package comprising a first and a second PTCR being thermally coupled therebetween. said first PTCR having a lower anomaly temperature than said second PTCR. and a selective heat removal means for providing stronger thermal coupling of said second PTCR than that of said first PTCR to the environment of said package.

2. The PTCR package of claim 1 further comprising a metal plate wherein said thermal coupling between said PTCRs is accomplished by mounting said first and second PTCRs to opposite sides of said metal plate. an extension of said plate serving as a common lead thereto.

3. The PTCR package of claim I further comprising a first and second metal plate being mounted to opposite sides of said first PTCR. a third and fourth metal plate being mounted to opposite sides of said second PTCR. and an electrically insulating layer. said thermal coupling between said PTCRs being accomplished by mounting said insulating layer between said second plate and said third plate. and said plates each having an extension serving as an electrical lead so that independent electrical access is provided to each said PTCR. 

1. A PTCR package comprising a first and a second PTCR being thermally coupled therebetween, said first PTCR having a lower anomaly temperature than said second PTCR, and a selective heat removal means for providing stronger thermal coupling of said second PTCR than that of said first PTCR to the environment of said package.
 2. The PTCR package of claim 1 further comprising a metal plate wherein said thermal coupling between said PTCRs is accomplished by mounting said first and second PTCRs to opposite sides of said metal plate, an extension of said plate serving as a common lead thereto.
 3. The PTCR package of claim 1 further comprising a first and second metal plate being mounted to opposite sides of said first PTCR, a third and fourth metal plate being mounted to opposite sides of said second PTCR, and an electrically insulating layer, said thermal coupling between said PTCRs being accomplished by mounting said insulating layer between said second plate and said third plate, and said plates each having an extEnsion serving as an electrical lead so that independent electrical access is provided to each said PTCR. 